In these examples of the prior art, the touch surfaces are configured so as to detect one or more contacts, which can be from a finger or by means of an accessory such as a stylus. When they comprise a plurality of sensors arranged according to a matrix organization, they allow the movement of said points of contact to be tracked.
The surfaces of the prior art are made functional, or functionalized, by means of sensors that measure the variation in a physical property during the contact with a finger or with an accessory on the touch surface.
The main physical properties measured, according the art, are resistivity or electrical resistance and the variation in electrical capacitance. There are also technologies based on the emission of ultrasounds or infrared rays.
The touch surfaces of the prior art can primarily detect the contact, but cannot quantify it. ‘Quantify’ means to determine the pressure or the resulting contact force, the direction of this resulting force when it comprises components other than normal to the touch surface, and the coordinates of the point of application of this force in a reference space linked to the touch surface, the contact forces applied to such a touch surface being essentially normal to said surface. According to certain embodiments of the prior art, said contact must be applied with a sufficient force to be detected, for example in the case of a measurement of resistivity or resistance, but does not give information about the intensity of the contact force with the surface. An example of this embodiment is described in US Published Application 2008/238882.
For some applications it can nevertheless be useful to have information about this force, or about the contact pressure exerted, about the points of application of the force, in particular when it is desired that the touch surface is able to detect several points or areas of contact, especially simultaneously, and lastly about the direction of the applied force.
The invention aims to create touch surfaces which response makes it possible to quantify the system of contact forces that are applied to them.
The technology known from the prior art allowing this need to be met consists of realizing a test specimen, i.e. a deformable solid whose modes of deformation under the effect of a loading system are known or can be calculated by direct methods, said test specimen being deformed under the action of the forces applied to it. Then its deformation just needs to be measured in order to know, by reverse method, the loading system to which it is subjected. The deformation of the proof body is measured most often by strain gages, which provide a variation in an electrical property proportional to their elongation. However, the difficulty with this method lies in several aspects. Firstly, for this method to be effective the shape of the test specimen must be simple and its supporting conditions controlled. As an example, while it is easy to determine by reverse method the loading mode of a thin flat test specimen held by its periphery when the forces normal to this plane are applied at points sufficiently far from the edges, it becomes significantly more complicated to apply such a method if the surface of the test specimen is curved or if it is held in a more complex way.
In addition, the test specimen must be sufficiently deformable so that its deformation can be measured by gages. However, in many applications mentioned above a rigid touch surface is desired. Therefore strain gages that are especially sensitive must be used. U.S. Pat. No. 7,116,209 describes especially sensitive strain gages suitable for a deformation measurement known as longitudinal, i.e. tangential to the surface on which they are bonded. These gages of the prior art are based, in their measurement principle, on the variation in the resistivity of an assembly of conductive nanoparticles when the distance between said particles is modified under the effect of the deformation of the test specimen on which said gages are bonded. Such gages can be networked to accurately measure the deformation of a test specimen in response to various loading systems. However, this principle suffers from the same limitations as the other technologies of the prior art, namely that the touch function, i.e. the ability to measure an action essentially normal to the surface, is dependent on boundary conditions, on the shape of the test specimen and on the ability to evaluate the latter's stressing by reverse method, the gages measuring the deformations tangential to the functionalized surface.